This invention relates to electric resistance heating elements, and more particularly, to polymer-based resistance heating elements for heating gases and liquids.
Electric resistance heating elements used in connection with water heaters have traditionally been made of metal and ceramic components. A typical construction includes a pair of terminal pins brazed to the ends of an Nixe2x80x94Cr coil, which is then disposed axially through a U-shaped tubular metal sheath. The resistance coil is insulated from the metal sheath by a powdered ceramic material, usually magnesium oxide. While such conventional heating elements have been the workhorse for the water heater industry for decades, there have been a number of widely-recognized deficiencies. For example, galvanic currents occurring between the metal sheath and any exposed metal surfaces in the tank can create corrosion of the various anodic metal components of the system. The metal sheath of the heating element, which is typically copper or copper alloy, also attracts lime deposits from the water, which can lead to premature failure of the heating element. Additionally, the use of brass fittings and copper tubing has become increasingly more expensive as the price of copper has increased over the years.
As an alternative to metal elements, at least one plastic sheath electric heating element has been proposed in Cunningham, U.S. Pat. No. 3,943,328. In the disclosed device, conventional resistance wire and powdered magnesium oxide are used in conjunction with a plastic sheath. Since this plastic sheath is non-conductive, there is no galvanic cell created with the other metal parts of the heating unit in contact with the water in the tank, and there is also no lime buildup. Unfortunately, for various reasons, these prior art, plastic-sheath heating elements were not capable of attaining high wattage ratings over a normal useful service life, and concomitantly, were not widely accepted.
This invention provides electrical resistance heating elements capable of being disposed through a wall of a tank, such as a water heater storage tank, for use in connection with heating a fluid medium. The element includes a skeletal support frame having a first supporting surface thereon. Wound onto this supporting surface is a resistance wire which is capable of providing resistance heating to the fluid. The resistance wire is hermetically encapsulated and electrically insulated within a thermally-conductive polymeric coating.
This invention greatly facilitates molding operations by providing a thin skeletal structure for supporting the resistance heating wire. This structure includes a plurality of openings or apertures for permitting better flow of molten polymeric material. The open support provides larger mold cross-sections that are easier to fill. During injection molding, for example, molten polymer can be directed almost entirely around the resistance heating wire to greatly reduce the incidence of bubbles along the interface of the skeletal support frame and the polymeric overmolded coating. Such bubbles have been known to cause hot spots during the operation of the element in water. Additionally, the thin skeletal support frames of this invention reduce the potential for delamination of molded components and separation of the resistance heating wire from the polymer coating. The methods provided by this invention greatly improve coverage and help to minimize mold openings by requiring lower pressures.
In a further embodiment of this invention, a method of manufacturing an electrical resistance heating element is provided. This manufacturing method includes providing a skeletal support frame having a support surface and winding a resistance heating wire onto the support surface. Finally, a thermally-conductive polymer is molded over the resistance heating wire to electrically insulate and hermetically encapsulate the wire. This method can be varied to include injection molding the support frame and thermally-conductive polymer, and a common resin can be used for both of these components to provide a more uniform thermal conductivity to the resulting element.